Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular constituents. Optical probes for such purposes are being employed in on-line process control in increasing numbers. These probes are often installed directly into the process stream or reactor, thus posing a potential safety hazard.
Free-space optical spectroscopy probes used in immersed applications typically involve imaging an optical sampling beam through a window bonded in the wall of a containment vessel. The window can be any material transparent to wavelengths of interest, though the most popular is sapphire. Typically the window is simply a flat surfaced window. As shown in FIG. 1, a lens 104 is commonly used behind the window 102 to focus the sampling beam into or onto the sample at 108. Another type of prior-art immersion optic on the market is shown in FIG. 2. In this case, a ball lens 210 serves the function of both the window and lens.
The amount of signal that can be obtained from the sample (particularly opaque samples) is related to the image quality of the sample beam at the focus and the numerical aperture NA. A higher NA yields a stronger, higher-quality signal. Also, the amount of data contamination from the signal caused by the window material is related to the numerical aperture of the sample beam and the type of aberrations it has.
FIG. 3 is a close-up diagram of the sample focus area for the flat window lens combination of FIG. 1. In this case, the lens is a diffraction-limited asphere which would produce a very high quality image absent the aberrating effect of the window. Other lenses, such including spherical optics could alternatively be used. The illustration shows the image is of reasonably high quality, but possesses a significant amount of negative spherical aberration due to the introduction of the window. With this type of aberration, the high NA components of the beam focus further away from the window than the low NA components. The low NA components have a greater depth of focus than the high NA components and therefore contribute more window signal contamination. The fact that the low NA components are close to the window exacerbates the signal contamination.
FIG. 4 shows the sample focus zone 212 for the ball lens of FIG. 2. In this case, there is a large amount of positive spherical aberration. This produces a relatively poor quality image and thus less overall signal, but due to the high NA components focusing close to the window and the low NA components further away, the window signal contamination is much less than with the flat window high quality lens combination.